what is the function of active directory domains and trust, active directory sites and services and, active directory users and computers in windows server 2003?
It is a user/computer manager, that includes everything about users logging to computers in Active domains. The purpose is to be able to change settings on one computer(server) and the change would be distributed among users. For example I prohibit screensavers change. And because of this all user would not be able to change it. You can restrict a user group to specific computer, so that no other would be able to logon to it. The trust is when you have like 2 companies and they merge/cooperate. So you don't need to copy all users, you just make a trust (trust them) and then you can manage their users and let them use your computers with their userID. And you can also manage computers(Workstations) rather then users. Like prohibit everyone logged to this certain (group of) computer the use of USB...
To centrally manage users and computer in company.
An example if you have 7 workstations (without Active Dir.) then on boss order you would have to run to each and restrict the use of something. Using AD you would just login to the AD server and restrict the use on a user/computer group in Active direstory. If you think in large numbers, then it makes a sence (20 and more).
You can use except of "domain" use company.
The Windows 2000 (server), 2003, 2008 have the additional packages that make it able to create an active directory.
I would suggest the use of google for more information.